In an effort to obtain information on the molecular organization of the mammalian urothelium as a basis for understanding biochemical events associated with differentiation and neoplastic transformation, we have succeeded in isolating the individual particles from the hexagonal plaques located in the asymmetric luminal plasma membrane. They have been separated in Triton X-100 and morphologically identified as rings of 12 nm in diameter. This achievement opens the following possibilities: a) a more precise chemical characterization of the subunit necessary to solve the molecular organization and function of this specialized membrane, b) to test the particles as site of enzymatic activity, particularly Na ion K ion dependent ATPase, and c) explore the type of interactions of this unit with the lipid bilayer. Time lapse cinematography and scanning electron microscopy has been used in an attempt to understand the behavior of the cell surface of primary cultures of dog urothelium, particularly in the stages of adhesion to substrate, migratory movements and cell to cell association. It seems that early cell-to-cell contacts immobilize surfaces of mammalian urothelium cells, resulting in a decrease of ruffling, and firm adhesion to substrate. Correlative studies of the critical step of urothelium culture has been performed with the transmission electron microscope, particularly in the early formation of permanent structures related with cell association, and formation of attachment points of cell to substrate. The study of the ultrastructure of human bladder carcinoma has failed to demonstrate unequivocal occurrence of virus particles. Tight and gap junctions are absent, and punctuate desmosomer is the only structure related with cell association that remains in the neoplastic tissue. Cell surfaces of separated neoplastic cell seem to be smooth as compared with the surface of cells isolated from normal urothelium.